UV laser radiation at wavelengths less than 200 nm and average power greater than 1 Watt (W) is useful in industrial applications such as laser machining, lithography, and optical inspection. Presently, the only laser types that will generate 1 W or more of such radiation directly, i.e., as the fundamental wavelength, are excimer and molecular fluorine (F) lasers. These lasers are very expensive to operate and maintain compared with other laser types such as diode-pumped solid-state (DPSS) lasers, including fiber lasers, which, unfortunately, have fundamental wavelengths at 900 nm or more.
Generation of sub-200 nm UV radiation from a DPSS laser having a fundamental wavelength greater than 900 nm requires that the fundamental output of the laser be frequency converted by frequency-doubling and sum-frequency mixing in a series of optically nonlinear crystals. In order to convert the output of such lasers having a wavelength of 1000 nm or more to a wavelength less than 200 nm, conversion would have to be to the sixth or higher harmonic. Harmonic conversion is limited, however, by the availability of optically nonlinear crystal materials that can transmit UV radiation less than 200 nm. A crystal of cesium lithium borate (CLBO) is presently the most preferred crystal for converting at wavelengths less than 200 nm, but even so, is limited to converting to wavelengths longer than about 190 nm.
Ytterbium-doped (Yb-doped) fiber lasers and neodymium-doped (Nd-doped) yttrium aluminum garnet (YAG) lasers have a fundamental wavelength of about 1064 nm. The sixth harmonic of this fundamental wavelength is about 177 nm, which is shorter than can be converted in CLBO. The fifth harmonic however is a wavelength longer than 200 nm. Erbium-doped (Er-doped) fiber-lasers can generate fundamental radiation at wavelengths between about 1510 nm and 1590 nm. The eighth harmonic (8H) of any of these wavelengths longer than 1520 nm would be less than 200 nm and within the conversion range of CLBO.
Schemes for generating the eighth harmonic of the output of an Er-doped fiber laser are disclosed in U.S. Pat. No. 6,590,698. In one conversion scheme disclosed therein, the second harmonic (2H) is generated in a first optically nonlinear crystal. The third-harmonic (3H) is generated in a second optically nonlinear crystal by sum frequency mixing the 2H-radiation with residual fundamental radiation. Fourth-harmonic (4H) radiation is generated by frequency doubling 2H-radiation in a third optically nonlinear crystal. A fourth optically nonlinear crystal sum-frequency mixes the 3H- and 4H-radiation to generate seventh-harmonic (7H) radiation having a wavelength of about 220 nm, and a fifth optically nonlinear crystal generates 8H-radiation (about 193-nm radiation) by sum-frequency mixing the 7H radiation with residual fundamental radiation.
As any sum-frequency mixing or frequency-doubling operation in an optically nonlinear crystal is at best only about 50% efficient, the overall conversion efficiency from a cascade of five such operations will be less than 3%. This would require a laser having a fundamental power of 32 W in order to provide UV (less than 200 nm) radiation having a power of more than 1 W. Clearly, there is a need for a more efficient scheme for generating sub-200 nm radiation by frequency conversion of the output of solid-state lasers.